Biometric recognition based on images of iris regions uses individual radial patterns in the iris. Image acquisition systems for such biometric recognition need to resolve at least 2 lines per mm within the iris pattern in order to provide reliable recognition of a subject. In mobile phones and smartphone in particular, there are two factors that limit direct use of conventional systems of lenses: the mobile phone market strives to minimize the overall camera cost; and making the phone housing as thin as possible.
It is desirable to have a dedicated image acquisition system that can capture images of the iris region of the face in sharp focus across a range of distances with approximately the same size and suitable for incorporation within the form factor of a smartphone.
More specifically, with a modern handheld device such as a smartphone, it is desirable to enable the user to hold the device at a distance that is comfortable for them, so that a user facing camera of the device can acquire a required image including an iris pattern. This distance could range from as close to the device as 15-20 cm to as far away as 40-45 cm and possibly further.
As well as to unlock a device, it may also be desired to acquire iris images during normal use of the device to continuously or periodically authenticate the user of the device—in such cases it would be undesirable to force the user to hold the device at any distance other than the distance at which they feel comfortable. Again this could range from, say, 15 cm up to and beyond 45 cm.
Typically iris regions for use in biometric recognition are acquired in near-infrared (NIR) wavelengths ranging from 740 nm up to 900 nm and even 1000 nm and beyond and the optical system needs to be designed and optimized accordingly. However it can be challenging to achieve an acceptably sharp focus depth of more than about 10 cm for lenses used in such systems. So, for example, a fixed focus lens that has a focus peak at, say, 20 cm, would only provide a usable range from 15-25 cm for iris acquisition.
A lens with variable focus might be employed, but this would result in changes in the size of the iris regions extracted. In addition, using an auto-focus mechanism may be prohibitive in terms of space or for cost constrained applications.
Referring to FIG. 1, the average distance between eyes varies, depending on sex and ethnicity, from about 60-65 mm and the average iris diameter ranges from about 10.1-13 mm. Also, as the eyes may not be exactly centered, it may be desirable to acquire a rectangular region of approximately 100 mm width to ensure both eyes are imaged. If we consider the field of view (FoV) of the camera to acquire suitable image width at a distance from the eye region to the face of d=15 cm we see that the half angle of the FoV is given as a tan(50 mm/150 mm) which gives a horizontal field of view angle of approximately 37 degrees.
With a variable focus lens, when the focus is switched to a longer distance, say 40 cm, the distance between the eyes and the size of iris pupil will be smaller in terms of pixels, as the face is more distant. An approximate estimate can be made based on an exemplary image sensor of 3,000 pixels by 1,800 pixels which is approximately a 5 megapixel sensor (although it will be appreciated that a range of sensor sizes and resolutions may be employed). At 15 cm we showed that this maps onto 100 mm width, thus the individual iris region with an average diameter of 12 mm will be 360 pixels across. However at 30 cm, the same iris region will be reduced to 180 pixels and at 40 cm it will be further reduced to 120 pixels or less—a point where the quality of the acquired image of the iris pattern may not be sufficient for recognition purposes (although some applications may require less than 120 pixels). It will also be appreciated that the illumination of an iris at 40 cm will be significantly less than at 15 cm and this can also cause problems with acquiring the required image quality at such distances.
Even then, if there is a need for a resizing of the iris region in each case to normalize iris images acquired at different distances to the same size of iris region before processing, such normalization may be a potential source of artifacts and errors in the underlying biometric pattern.
Further, in an auto-focus system, the user must hold the device in a fixed position until a focus lock is achieved. This can take several seconds and can provide a less than optimal focus depending on the number of focus stops provided by the focus mechanism.
Finally, it will also be appreciated that it can be useful to be able to acquire an image of both a face surrounding an iris region as well as the iris region for tracking purposes. Thus, a face detection and tracking system could acquire images continually and locate an iris region without the IR illumination typically used to acquire an image of an iris pattern. However, using a single auto-focussing system would make it difficult to acquire images of both iris and face regions across a range of 15 cm to 40 cm.
It can be seen therefore that the design of an image acquisition system for iris regions that can capture both iris regions at the same time and across a range of distances suitable for a modern handheld imaging device such as a smartphone presents some challenges.
CN101154264 discloses a large field depth iris image acquisition and recognition system based on a plurality of fixed-focus cameras. The system images the iris with the fixed-focus cameras each having different imaging object distance ranges and produces multi-path fixed-focus image results. One or a plurality of multi-path fixed-focus image results is chosen through distance measurement or image quality evaluation method, before image processing and identification of chosen qualified iris image data is performed. Each of the cameras employed however comprises the same optics, set to a respective focus distance and so this system involves many of the drawbacks highlighted above.